The present invention relates to a power unit assembly for a commercial vehicle.
Power unit assemblies of the type comprising an internal-combuston engine, a mechanical gearbox, a clutch interposed between the engine and the gear change are known, in which the gearbox, the relevant synchronisation and the control of the injection pump for the engine are controlled by an electronic processor.
In power unit assemblies of the type briefly described above, the injection pump is usually provided with mechanical control means or it is controlled electronically so that the engine speed is maintained within a range of values between a lower or "minimum" limit, for example 600 rpm (below which the engine would stall), and an upper limit coinciding with the speed at maximum power of the engine itself.
Moreover, before effecting the engagement of the gear ratio required by the driver, said electronic processor normally carries out checks to verify whether said gear ratio is consistent with a speed range in which it is desirable for the engine itself to operate.
In particular, the control logic programmed into the processor usually prevents engine speeds from being attained, at the end of the gear-change operation, which are below the maximum torque speed (for example, equal to 1200 rpm) for the operations of changing to a higher gear ratio, or speeds above the maximum speed of rotation allowed by the injection pump (for "climbing" gear-change operations).
This logic fulfills the two aims of maintaining the engine within the predetermined speed range and of ensuring synchronisation of the gear change. In fact, if the management logic of the processor does not prevent the engagement of a gear ratio corresponding to an engine speed which is higher than the above-defined maximum speed, as a result of the maximum control exerted on the injection pump the engine would not be able to accelerate the gear masses to the synchronisation speed required by the gear ratio selected, thereby rendering impossible or even dangerous the engagement of the gear ratio itself.
To sum up, known power unit assemblies of the type briefly described above do not enable gear changes to be effected if, after this operation, the engine speed is above or below the normal operating range.
This is certainly an advantage under normal driving conditions; however, in certain driving conditions it may represent a dangerous drawback because it is not possible to carry out emergency gear changes which, on the other hand, are possible with conventional power unit assemblies having a gearbox with mechanical synchronisation. For example, one such case is the engagement of a very low gear ratio to utilise the braking action of the engine when descending a steep slope or in the event of failure of the braking system.